The present invention relates to a plastic coupling for plastic tubing for connecting two tubes. For example, the semiconductor manufacturers use a great amount of various chemicals with a great amount of pure water. With automation of the manufacturing process, it is required to move the chemicals and pure water through a great variety of measures such as pumps, pressurized gas and the like, depending on the purpose. Therefore, they must use tubes and joints made of fluoroplastics which are stable to chemicals and resistant to heat.
It is essential that piping members for these tubes and joints are not simply disconnected under the temperature and pressure of the liquid being transferred. To meet this requirement, various types of joint structures have been proposed and practically used.
FIGS. 9 and 11 show a plastic coupling used in the prior art. The plastic coupling 50 comprises a main joint body 52 having a central fluid passages 51, a pair of tube fixing nuts 54 screwed over the joint body 52 at the opposite ends, each of said tube fixing nuts 54 having a tube end receiving bore 53 aligned with the fluid passage 51 of the joint body 52, and a plastic connection tube 56 having an enlarged diameter portion 55 with a flared tip end.
When the connection tube 56 is to be connected to the joint body 52, the sleeve portion 52a of the joint body 52 is first sealingly fitted into the enlarged diameter portion 55. The nut 54 is then removed to a position in which it covers the enlarged diameter portion 55. The female or internal thread 54a of the nut 54 is further screwed onto the male or external thread 52b of the joint body 52.
When the female thread 54a is fully screwed onto the male thread 52b, the shoulder of the reduced diameter portion 54b in the nut 54 is engaged by the tapered portion 55a of the enlarged diameter portion 55 while the tapered portion 55a is urged against the tip tapered portion 52c of the sleeve portion 52a. Thus, the tapered portion 55a will be clamped between the joint body 52 and the nut 54.
Although not shown in FIG. 9, another similar tube 56 will be mounted in the right-hand end of the joint body 52.
In the plastic coupling of the prior art, however, the connected tubes 56 may easily be removed out of the joint body 52 when the tubes are pulled in the longitudinal direction. Particularly, when the joint body 52 is formed of fluoroplastic and if it is used in transfer of any liquid having a temperature equal to or higher than 100.degree. C., the sleeve portion 52a of the joint body 52 tends to be inwardly deformed as shown in FIG. 10 because the fluoroplastic material has a very large expansion coefficient. With such a deformation, the clamping force by which the connection tube 56 is held between the joint body 52 and the nut 54 will be reduced. If a large external pulling force is applied to the connection tube 56 in the longitudinal direction (as shown by arrow A in FIG. 10), it may easily be removed out of the joint body.
In order to overcome the above problem, an improvement has been proposed which as shown in FIG. 11, the tapered portion 55 of the connection tube 56 is formed with a groove 57 while another reduced diameter portion 54c is formed in the inner wall of the nut 54 at a position corresponding to the groove 57. When the shoulder of the reduced diameter portion 54c in the nut 54 is engaged by the groove 57, a further engagement for preventing the tube from being removed out of the joint body may be provided.
However, such a further engagement cannot fully prevent the tube from being removed out of the joint body when the latter is used in the increased temperature as mentioned. When the engaging groove 57 is formed in the tapered portion 55, the flared tube portion will be reduced in wall-thickness and also in strength. This also raises another problem in that the flared tube end tends to be cracked by any applied load. FIG. 11 shows such a cracking by 58.